The present invention relates to a method for emptying a reducing agent delivery system of an SCR catalytic converter. In addition, the present invention relates to a computer program which executes each step of the method according to the invention when it runs on a computing device, and a machine-readable storage medium which stores the computer program. Finally, the invention relates to an electronic controller which is configured to carry out the method according to the invention.
Nowadays, during the post-treatment of exhaust gases, the SCR (Selective Catalytic Reduction) method is used in order to reduce nitrogen oxides (NOx) in the exhaust gas. The SCR method is primarily used in motor vehicles. Here, a urea-water solution, commercially also known as AdBlue®, is introduced into the oxygen-rich exhaust gas. For this purpose, use is made of a metering module which comprises a nozzle in order to spray the urea-water solution into the exhaust gas stream. In addition, a metering valve is implemented, which is able to control the flow through the metering module. In the SCR catalytic converter, the urea-water solution reacts to form ammonia, which then binds to the nitrogen oxides, from which water and nitrogen are produced. The urea-water solution is pumped from a tank to the metering module via a delivery module. In addition, the delivery module is connected to the tank via a return, so that excess urea-water solution can be fed back. The urea-water solution must be removed from the system at the end of the journey since otherwise, above all at low outside temperatures, there is the risk of freezing the urea-water solution, which can lead to damage to the delivery module. Normally, the delivery module is emptied into the tank via the delivery line. In this case, the ventilation is carried out via the metering module and the pressure line. However, in the first seconds of the emptying process, the water component of the urea-water solution can evaporate, which leads to the urea crystallizing out. This can result in blockage of the metering module, in particular of the metering valve. If this is the case, the emptying of the delivery module via the pressure line is not possible and pressure equalization cannot be completed.
DE 10 2007 028 480 A1 describes an SCR device as presented above. Here, a second storage container for a reducing agent is additionally connected and is connected up as soon as the first storage container falls below a minimum filling quantity. The fluid is delivered to the metering module via a pump and can run back into the storage container via a return. The system manages without valves in the lines listed.
DE 102 54 981 A1 relates to a device for removing a reducing agent from an SCR system. The system has an additional compressed air pump and also two control valves and a return line. During emptying, in a first step the compressed air pump is connected via a control valve such that it first empties the metering valve, either into the mixing chamber of the exhaust gas tract or into the reducing agent reservoir. In the second step, the compressed air pump is connected via both control valves in such a way that the compressed air pump is able to empty the reducing agent line and the return line into the reducing agent reservoir. The arrangement in the second step is independent of the metering module. The device thus permits the emptying of a major part of the system, irrespective of the state of the metering module.
DE 10 2013 210 858 A1 relates to a method for operating a hydraulic delivery and metering system, in particular for a reducing agent in an SCR system which has a delivery path and a separate return path. There is a pump and two nonreturn valves each in both paths. When emptying the system, the return pump sucks the delivery path and the metering module empty. In addition, it is mentioned that, in the event of a defect in the return path, the pressure equalization can take place via the metering valve.